1. Field of the Invention
The present invention relates to a hybrid vehicle and to a method and system for shutting down an engine in a hybrid vehicle to improve the noise, vibration and harshness (“NVH”) characteristics of the engine shutdown event.
2. Description of the Related Art
The motor vehicle industry is actively working to develop alternative powertrain systems in an effort to improve vehicle fuel economy and reduce the level of pollutants exhausted into the air by conventional powertrain systems equipped with internal combustion engines. Significant development efforts have been directed to electric and fuel-cell vehicles. Unfortunately, these alternative powertrain systems currently suffer from several limitations and, for all practical purposes, are still under development. However, “hybrid” vehicles, which typically include an internal combustion engine and an electric motor and/or generator, offer a compromise between traditional internal combustion engine powered vehicles and full electric powered vehicles.
Hybrid vehicles are generally classified as either series hybrid vehicles or parallel hybrid vehicles. In a series hybrid vehicle, a generator is driven by the mechanical output of an internal combustion engine. The output of the generator may be combined with the output of a vehicle battery to drive an electric motor, which in turn drives the vehicle.
Parallel hybrid vehicles, on the other hand, are usually driven directly by the mechanical output of the internal combustion engine. However, when the vehicle must be accelerated or decelerated at a rate that cannot be accomplished by the internal combustion engine alone, the electric motor-generator, which is mechanically connected to the internal combustion engine, operates as an electric motor (on acceleration) or as an electric generator (on deceleration) to meet the required rate of acceleration or deceleration through the combined output of the internal combustion engine and the electric motor-generator.
In certain hybrid vehicle configurations, the engine is shut down periodically during vehicle operation when power is not required to conserve fuel and reduce emissions. During engine shutdown, the pumping processes within the engine result in torque disturbances and engine crankshaft speed oscillations that adversely effect the smoothness of the engine shutdown event. Noise, vibration and harshness associated with the shutdown event is generally undesirable since it may be perceived by the vehicle occupant.